Cavity pellet emissive electrode

ABSTRACT

An emissive electrode of the type utilized in fluorescent lamps having a conic section cavity formed therein, said electrode having a bulk density gradient structure. The electrode includes a fused pellet and a metal lead extending therefrom. The pellet contains a fused mixture of electron emissive material and a metal having a high melting point and a low vapor pressure at the operating temperature of the lamp.

United States Patent l H Butter et al. [4 1 Feb. 27, 1973 541 CAVITY PELLET EMISSIVE 1 References Cited ELECTRODE UNITED STATES PATENTS [751 lnvemmsl Wald Beverly M3554 3,482,138 12/1969 Vollmer ..313 311 Edmund R. Kern, Hampton, N.H.; Richard Menelly, Danvers, Mass- Primary Examiner--David Schonberg Assistant ExaminerPaul A. Sacher [73] Assignee. International Telephone and Tele Atwmey C' Come Remsen, Jret al.

graph Corporation, Nutley, NJ. [22] Filed: March 31, 1972 [57] ABSTRACT [21] Appl, No; 240,110 An emissive electrode of the type utilized in fluorescent lamps having a conic section cavity formed therein, said electrode having a bulk density LS. Cl. R, 1 gradient st uctu e The electrode includgs a fused pel- [51] Int. Cl ..H01j 1/30, HOlj 1/38 let and a metal lead extending therefrom. The pellet [58] Field of Search ..3l3/3l l, 346 R, 346 DC; contains a fused mixture of electron emissive material 1 29/2517 and a metal having a high melting point and a low vapor pressure at the operating temperature of the lamp.

10 Claims, 4 Drawing Figures 700 PR/OR ART ELECTRODES HAVING BUAK 2 DENSITY GRAD/f/VT STRUCTURES o 600 E O k R 3 500 S3; 400 W! Y1, 30o cAv/rr use moo: g? 200 I 100 e 2 l l l l l 1 J 300 600 900 1200 I500 I800 ai'oo e400 2700 5600 BURNING TIME (HOURS) 1 CAVITY PELLET EMISSIVE ELECTRODE BACKGROUND OF TIIE INVENTION This invention relates to cold cathodes, emissive fused pellet electrodes having bulk density gradient structures and, more particularly, to such electrodes having a cavity formed therein.

Emissive electrodes are utilized in fluorescent lamps to supply free electrons thereby enabling current flow in the fluorescent tube and may, therefore, be called cathodes.

The cathodes normally comprise one or more of the alkaline earth metals and compounds thereof as these materials have relatively low work functions and are therefore able to supply a copious quantity of free electrons without requiring the expenditure of great amounts of energy. The operation of the cathode will, of course, deplete the electrode material and when the material is depleted to the point where the electrode can no longer supply sufficient electrons for lamp operation upon the application of standard fluorescent lamp voltages, the lamp will fail and will have to be discarded. It is therefore clear that it is advantageous to provide emissive electrodes incorporating the greatest amount of emissive material possible and to design these electrodes so that they retain the emissive material for the greatest time possible.

The cathodes presently utilized in the art are normally one of two types both of which are, for operation, heated to what is termed the thermionic emission temperature, at which temperature they emit electrons. The first of these cathodes is heated to its emission temperature by a heated filament and is therefore termed, for the purposes of this specification, a hot" cathode,

while the other of said cathode types is heated to its emission temperature by ionic bombardment and is therefore termed, for the purposes of this specification a cold cathode.

The instant invention relates to cathodes of the cold type and a brief general discussion of this type of cathode is provided here as an aid to understanding the invention.

The philosophy behind the development of the cold cathode is to provide a large quantity of electron emissive material, for example 50 milligrams of alkaline earth material, within a container. Cathodes of this type, being described for example in U. S. Pat. Nos 2,677,623, 2,753,615 and 3,325,281, are so termed because, as stated above, they are not provided with a heating filament for direct cathode heat as are hot cathodes. Rather, the cold cathodes are ignited or driven into their thermionic emission mode by the provision of a relatively high ignition voltage, approximately 500-550 volts across the lamp electrodes. The ignition voltage ionizes the atmosphere in the fluorescent lamp, said atmosphere usually being a combination of an inert gas, such as argon, at a pressure of approximately 2.5 to 3 millimeters and mercury vapor at a pressure of approximately 9 microns. The ions thus provided impinge upon the emissive surface of the cathode with sufficient energy to heat the surface, thereby causing it-to become electron emissive.

To overcome the deficiencies of the cold cathodes described in the aforementioned U. S. Patents, improved cold cathodes having density gradient structures, and described in copending U. S. Pat. applications Ser. Nos. 204,469 and 204,478, assigned to the assignee of the instant invention, were developed. However, these last-mentioned cold cathodes were also found to be unsatisfactory in that their ignition voltage was found to increase rapidly after a short burning period. Thus, it has been discovered that the ignition voltage of these electrodes tended to increase from approximately 400-450 volts to 500-550 volts after only several hundred hours of burning. Use of this type of cold cathode in a fluorescent lamp results in a lamp that is obviously unsatisfactory in that burning of the lamp for a relatively short period of time results in a lamp ignition voltage requirement which exceeds the ballast capabilities of presently utilized lamp fixtures. It is believed that the reason for the increase in ignition voltage is two-fold. Firstly, a portion of the electrodes emissive material is discharged into the lamp atmosphere during the glow to are transition stage due to the aforementioned ion bombardment, and secondly, an appreciable amount of the cathodes surface emissive material is driven into the interior of the electrode where it is no longer available to support ignition due to temperature differentials in the cathode structure. These two factors combine to result in a cathode having a surface poor in emissive material and therefore a cathode which requires greater and greater voltages to pass from the glow discharge stage to the arc discharge stage.

SUMMARY OF THE INVENTION Therefore the main object of this invention is to provide an improved emissive cathode having a bulk density gradient structure which maintains a constant ignition voltage over an extended period of time.

It is a further object of this invention to provide such a cathode which maintains a surface rich in emissive material for an extended period of time.

According to the present invention there is provided a fused pellet electrode having a top, a bottom and sides and containing a fused mixture of electron emissive material and a metal, said pellet having a cavity, the cross-section of which is a conic section, formed in the top thereof, and a conducting lead embedded in and extending fromsaid pellet, said pellet having a bulk density gradient structure wherein the lower portions of said pellet have a high bulk density relative to the bulk density of the upper portions thereof.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the apparatus utilized in constructing the inventive electrode;

FIG. 2 illustrates a cross-sectional view of the inventive electrode;

FIG. 3 illustrates the inventive electrode connected in a fluorescent lamp; and

FIG. 4 provides a graphical representation of a lamp burning time versus lamp ignition voltage for the instant electrode and for prior art fused electrodes having bulk density gradient structures.

DESCRIPTION OF THE PREFERRED EMBODIMENT An example of the method and apparatus utilized in constructing the subject fused pellet electrode will now be described.

In the construction of the instant electrode a mixture of a powder of a metal having a high melting point, over l,400C, and a low vapor pressure, such as a refractory metal or a transition-element metal, hereafter referred to as a transitional metal, and a source of electron emissive material is prepared. In the specific example to be provided below, a few grams of powdered tantalum, which is a refractory metal, approximately 99.8 percent pure, is mixed 60 percent by weight with reagent grade barium peroxide powder, 40 percent by weight. Of course, other refractory metal powders, such as tungsten, molybdenum, thorium, titanium, zirconium and mixtures thereof could be used instead of tantalum powder, while other electron emissive materials, for example, alkaline earth metal compositions such as oxides, peroxides, and nitrates of barium, and oxides, peroxides and nitrates of barium in combination with calcium oxide, strontium oxide and zirconium dioxide, or alkali metal compositions, such as compounds of lithium, cesium, sodium and potassium, could be used instead of barium peroxide. Mixtures of calcium, strontium and zirconium compounds are generally added to barium compounds in order to retard the loss of barium from the finally formed fused electrode.

It is appropriate to note at this point that in forming the subject fused pellet electrode it is necessary to control the rate of the exothermic reaction which is used to form the electrode, said reaction to be more fully discussed below, since if the rate of the reaction is too great much of the material utilized in forming the electrode will splatter off, resulting in its being lost. If, on the other hand, the rate of reaction is too slow, the density gradient of the resultant electrode will be too small, and the electrode structure will approach homogeneity. Such a homogeneous structure will result in a less efficient cathode than is otherwise obtainable. Further, if the rate of the exothermic reaction is too slow the resultant electrode will have a higher ignition voltage requirement than is otherwise obtainable since it will require more time to go through the glow to are transition stage due to the fact that it will have a more uniform surface, that is, it will have less protrusions available for starting the required arc.

At this time it is well to note that although, as stated above, different refractory metal powders, such as thorium or titanium may be utilized to form the instant fused electrode, these refractory metal powders have a greater exothermicity than, for example, tantalum. Thus, while an electron emissive material, such as barium peroxide, may here too be utilized in forming the subject cold cathode, it has been found advantageous to use a greater quantity by weight of, for example, zirconium powder and a lesser quantity of barium peroxide or other alkali or alkaline earth material than would be used if the refractory metal powder was, for example, tantalum. A satisfactory reaction rate may also be obtained if it is desired to use the same quantity by weight of zirconium powder as of tantalum powder, if the less reactive oxide of barium or other alkaline earth metal is substituted for the more reactive peroxide of the alkaline earth metal. In the same manner it has been found that transitional metals, such as nickel and iron, which have a lower exothermicity than tantalum may also be utilized to form the subject cold cathode. This may be accomplished by either utilizing a greater portion of alkali or alkaline earth compound such as barium peroxide than would be used if a refractory metal were used rather than the transitional metal, or alternatively, the same result may be accomplished by keeping the ratio of the transitional metal constant and using a more reactive compound of alkali or alkaline earth metal than barium peroxide, such as for example, barium nitrate. In general, although the above example specified that 60 percent by weight of a refractory metal powder and 40 percent by weight of an alkaline earth compound be used, it has been found through experimentation that ranges of mixtures of approximately 40 percent to percent of metal powder by weight and approximately 20 to 60 percent alkaline earth compound by weight may be utilized to produce satisfactory reaction rates and therefore may be used to produce satisfactory fused electrodes.

Returning now to the example, the above described mixture is prepared by rolling six parts by weight of tantalum powder and 4 parts by weight of barium peroxide with flint pebbles in a standard porcelain jar mill, for a period of, for example, 1 hour.

Referring now to FIG. 1, the mixed powder 1 is placed in recess 2 of a split mold 3 formed of sections 4 and 5 and which may be made of cast iron, steel or ceramic. In this particular example type 316 steel is utilized since this material readily becomes heated by a radio frequency field and the reacting materials do not adhere thereto. Recess 2 is parabolic in shape and has an upper diameter of approximately 0.2 inches and a depth of approximately 0.25 inches. Approximately 280 to 300 milligrams of powder mixture 1 is placed in recess 2 and it is then compressed by, for example, a weighted steel plunger, not shown, with a pressure of approximately 500 to 2500 pounds per square inch. It is appropriate here to note that although superior electrodes are constructed using the aforementioned pressures, it has been discovered that operable electrodes may be constructed using little or no pressure. A mold cover 10, which may be made of the same material as mold 3, is positioned atop said mold and protrusion 11, integrally formed with said cover 10, extends into said powder mixture 1. The shape of protrusion 11 has been designed to approximate a conic section, that is, the cross-section of said protrusion is defined by the equation: at 2hxy by cx dy e 0. This equation is satisfied by five shapes, these being an ellipse, a circle, a hyperbola, a parabola and two straight lines. In this example, however, the shape illustrated is a parabola although it must be understood that this is merely by way of example as was the case also with regard to the shape of recess 2 which also preferably has the crosssectional configuration of a conic section for reasons which will be discussed below. Hemispherical indentations 12 and 13 are formed in sections 4 and 5, respectively, of mold 3 and conducting lead 14, having a curved head as illustrated, which may be made of a suitable conducting material such as nickel, tungsten, tantalum, iron and alloys thereof, is positioned within the hole formed by indentations 12 and 13, the curved head of lead 14 being positioned within powder mixture 1 in recess 2.

it is appropriate here to note that the pressure with which mixture 1 is compressed is, while not critical, as evidenced by the fact that the suitable compression pressure may range between 500 and 2500 pounds per square inch, important. This is true notwithstanding the fact that, as above stated, operable electrodes may be constructed utilizing little or no pressure. As discussed above, the rate of exothermic reaction controls the density gradient of the completed electrode and it will be clear to those skilled in the art that the degree of compression to which the powder mixture is subjected will affect the rate of the aforementioned exothermic reaction.

Mixture 1 may now be heated to initiate the desired exothermic reaction between the tantalum and barium peroxide. The heat necessary to start said exothermic reaction may be provided in a number of ways, for example, by a muffle furnace. It may also be provided by the structure here illustrated which includes an rf work coil 15 surrounding mold 3, said coil 15 being connected to a source of electrical energy by conductors 21 and 22. To begin the exothermic reaction it is necessary to heat mixture 1 to a temperature between 700C and 1,000C, said temperature being above the melting temperature of the barium peroxide powder and the temperature at which the exothermic reaction will begin within the mold. To provide the required heating, taking into account the impedance of the mold, and the material which is to be heated, the source of electrical energy 20, here illustrated, is selected to operate at a frequency of 450 kilohertz and to provide a current of approximately 165 milliamperes. Source 20 must remain energized until the exothermic reaction begins. Once the exothermic reaction begins source 20 may be de-energized since the exothermic reaction will continue until it self-extinguishes, the duration of the reaction being determined by the quantity of the mixture 1 present within mold 3. After the exothermic reaction has ended and the completed fused pellet electrode has been cooled, it is available for activation and subsequent use in a fluorescent lamp.

Referring now to FIG. 2, there is illustrated the completed fused pellet cold cathode 25 having a cavity 26 formed therein, said cavity having the cross-sectional configuration of a conic section, which is here illustrated as a parabola, the axis of symmetry of the cavity being coincident with the axis of symmetry of the cathode. Cavity 26 has an upper diameter of approximately 0.1 inches and a depth of approximately 0.15 inches, said dimensions corresponding, of course, to the dimensions of protrusion 11. Pellet 25 has a bulk density gradient structure, by which is meant that the top portion 27 of the cathode contains between one and one and one-half times the void volume than that contained by the bottom portion 28 and deeper interior portions 29 thereof.

The particular structure and configuration of cathode 25 is due to the size and shape of the compressed powder mixture 1, the fact that it was closely confined within recess 2 during the occurrence of said exothermic reaction, the contour of protrusion 11 formed on the underside of cover 10, and of course, the outwardly directed force provided by said exothermic reaction.

Electrode 25 has been experimentally determined to have an ignition voltage of between 415 and 425 volts over an extended period of burning time as opposed to the fused pellet electrodes having bulk density gradient structures described in the above-mentioned U. S. patent applications. While the reason for this is not known with certainty, a theory presently accepted as correctly explaining the operation of the instant cathode will now be provided.

The shape of the cavity, that is, a conic section here illustrated as a parabola, concentrates the electric field within the cavity at the apex thereof, and this aids in the rapid transition from the glow discharge stage to the arc discharge stage. Thus, it is seen that the purpose of the cavity shape is to maximize the electric field intensity within cavity 26. The glow discharge, which is characterized by a diffuse are, a high cathode fall (approximately 100 to 200 volts) and a macroscopic dark space lasts approximately one-half a second, at which time the cathode enters the arc discharge stage which is characterized by a constricted discharge, a 15-20 volt cathode fall and a microscopic dark space. The are is struck within cavity 26 and then for a number of reasons, one of which is likely to be ambient gas convection currents, the arc climbs out of the cavity over the lip area of cathode 25 and in a random manner reaches a random point on the exterior surface of electrode 25. The fact that the arc is struck within cavity 26 and then travels to a point on the exterior surface of the cathode overcomes one problem which arises when prior art pellet electrodes having bulk density gradient structures, such as those described in the aforementioned copending U. S. Patent Applicatons, are utilized. This is due to the fact that during the glow discharge stage percent of the current flow in the lamp is due to rapidly travelling heavy ions, said ions dislodging emissive material, i.e., barium, from the cathode, this dislodged barium being driven into the ambient atmosphere of the fluorescent lamp tube where it is no longer useful in initiating lamp operation. Further, due to temperature gradients, that is, due to the fact that the temperature at the point at which the arc is formed is at a substantially higher temperature than the temperature existing at points in the interior of the cathode, barium is forced from the cathode surface into the interior of the cathode, thus rendering the cathode surface poor in emissive material which, in turn, contributes to the rise in ignition voltage in the lamp in which the cathode is used. The instant cathode, on the other hand, overcomes both these deficiencies in, it is believed, the following manner. Firstly, during the glow discharge stage any barium dislodged from the surface of the cavity due to the impingement of high speed ions will simply be redeposited at other points in the cavity and thus will not be driven from the cathode surface and into the ambient atmosphere of the fluorescent lamp tube. Secondly, due to the fact that the walls of the instant cathode are made very thin, in the order of 0.05 inches, the barium may theoretically be treated as a gas following a Maxwellian distribution. The temperature gradient existing between the surface of the cavity and the outer surface of the cathode caused by the heat due to the arc discharge will result, when the arc is within cavity 26, in the movement of barium from the cavity surface to the outer surface of the electrode. This in turn results in a barium rich exterior cathode surface which will easily support the are which, as previously stated, moves from the interior of the cavity to the exterior surface of the electrode. This of course results in the retention of the barium in a useful form as opposed to the condition existing with regard to the electrodes described in the copending applications wherein, as stated above, the barium was driven to the interior of the electrode where it was no longer useful. Conversely, when the arc is located on the outer surface of the electrode, that point at which it is located is at a substantially higher temperature than the temperature of the walls of the cavity. This results in a temperature gradient which tends to move the barium from the outer surface of the electrode toward the walls of the cavity and this results in a cavity wall surface rich in barium during lamp operation. When the lamp is shut off the temperature of the cathode drops extremely rapidly, the temperature decay having been measured at the instant of lamp shut off to be at the rate of approximately l0,000C per second. This in turn locks" or freezes the barium distribution into the configuration existing during lamp operation rather than allowing a uniform distribution of the barium which would be likely to result if the cooling of the electrode were more gradual. It will be clear to those skilled in the art that the result of the foregoing is that when the lamp is shut off there results a cavity wall rich in emissive material and one which is therefore well adapted to support a rapid glow discharge to arc discharge transition with a minimal ignition voltage.

Turning now to FIG. 3, there is illustrated a fluorescent lamp 30 of a type well known in the art having mounted therein the inventive cathode 25. Cathode 25 is connected to a mounting stem 35 positioned within the lamp by lead 36 and which is connected by for example, welding, the end of conducting lead 14.

Turning now to FIG. 4, there is illustrated a graphical representation of lamp ignition voltage requirements versus burning time for both the subject cavity pellet electrode and for electrodes of the type described in the aforementioned copending U. S. Patent Applications. Examination of FIG. 4 illustrates the fact that after approximately 150 hours, which may be termed the seasoning time, the ignition voltage of the instant electrode will have stabilized at approximately 400-425 volts and it will remain at this level for an extended period of time, this period of time already having been experimentally determined to exceed 3,000 hours. It will be seen, however, that the ignition voltage of those cathodes described in said copending applications increases with use until an ignition voltage requirement is reached after only several hundreds of hours of burning where the lamp in which such electrodes are utilized can no longer be ignited utilizing ballast circuits of the type presently in commercial use.

It will thus be seen that there has been provided an emissive fused pellet electrode suitable for use in fluorescent lamps which is superior to those cathodes presently known in the art due to the fact that it maintains a surface rich in emissive material for an extended period of time.

The subject cathode will of course be activated prior to its incorporation into a lamp and it is here appropriate to note that the instant structure has been discovered to be relatively air stable subsequent to such activation. The subject cathode will remain activated for a period of at least 1 hour when maintained after said activation in reasonably dry air, by which is meant air containing less than grains of water per pound of dry air. Thus, the cathode here described, while being greatly improved over the cathodes known in the art, for reasons previously discussed, additionally is suitable for batch processing, thus providing an additional valuable advantage.

While the principles of the invention have been described in connection with specific structures, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and vin the accompanying claims.

We claim:

1. A fused pellet electrode comprising:

a fused pellet having a top, a bottom and sides and containing a fused mixture of electron emissive material and a metal,

said pellet having a cavity, the cross-section of which is a conic section, formed in the top thereof; and

a conducting lead embedded in and extending from said pellet,

said pellet having a bulk density gradient structure wherein the lower portions of said pellet have a high bulk density relative to the bulk density of the upper portions thereof.

2. A fused pellet electrode, according to claim 1, wherein the axis of symmetry of said cavity is coincident with the axis of symmetry of said electrode.

3. A fused pellet electrode, according to claim 2, wherein the cross-section of said cavity is parabolic.

4. A fused pellet electrode, according to claim 3, wherein the cross-section of said pellet is of a conic section.

5. A fused pellet electrode, according to claim 3, wherein said metal comprises a refractory metal.

6. A fused pellet electrode, according to claim 3, wherein said metal comprises a transition element metal.

7. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkaline earth metal compound.

8. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkali material compound.

9. A fused pellet electrode, according to claim 3, wherein said fused pellet results from an exothermic reaction in a powder mixture comprising a powder of said metal and a powder of said electron emissive material.

10. A fused pellet electrode, according to claim 9, wherein said metal powder comprises tantalum powder and said electron emissive material comprises barium peroxide powder,

said tantalum powder being 60 percent by weight of said powder mixture and said barium peroxide powder being 40 percent by weight of said powder mixture.

i i k 

2. A fused pellet electrode, according to claim 1, wherein the axis of symmetry of said cavity is coincident with the axis of symmetry of said electrode.
 3. A fused pellet electrode, according to claim 2, wherein the cross-section of said cavity is parabolic.
 4. A fused pellet electrode, according to claim 3, wherein the cross-section of said pellet is of a conic section.
 5. A fused pellet electrode, according to claim 3, wherein said metal comprises a refractory metal.
 6. A fused pellet electrode, according to claim 3, wherein said metal comprises a transition element metal.
 7. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkaline earth metal compound.
 8. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkali material compound.
 9. A fused pellet electrode, according to claim 3, wherein said fused pellet results from an exothermic reaction in a powder mixture comprising a powder of said metal and a powder of said electron emissive material.
 10. A fused pellet electrode, according to claim 9, wherein said metal powder comprises tantalum powder and said electron emissive material comprises barium peroxide powder, said tantalum powder being 60 percent by weight of said powder mixture and said barium peroxide powder being 40 percent by weight of said powder mixture. 